The invention relates to a fuel injection device operating according to the solid-state energy storage principle, in particular for two stroke engines according to the preamble of claim 1.
Fuel injection devices which operate according to the solid-state energy storage principle are described in EP 0 629 265, in particular with reference to FIGS. 13 to 19. They operate according to the so-called pump stroke and nozzle principle with pressure surge injection, an initial accelerated partial stroke of an armature which acts as a delivery plunger, extends axially on one side and has an electromagnetically driven injection pump is provided, in which armature delivered fuel in the pump system is displaced without a buildup of pressure in the fuel fluid. During this initial partial stroke, the delivery plunger and/or the armature absorbs kinetic energy and stores it, the predetermined flow space, which is ensured by a fuel circuit in the pump system, being made available to the fuel which is displaced in the process. As a result of a sudden, predetermined interruption of the fuel circuit during the resistance-free pretravel of the delivery plunger and owing to the subsequent movement of the delivery plunger, said interruption being brought about by means of a valve device which is arranged in the armature and/or the delivery plunger and is activated by the armature movement, the delivery plunger imparts its stored kinetic energy in a sudden, pressure surge-like fashion to the quantity of fuel which is located in a spatial area of the circuit spacexe2x80x94the so-called pressure spacexe2x80x94between, and/or in, the delivery plunger and an injection nozzle which is closed off, for example in a spring-loaded fashion, said spatial area being formed by the interruption in the circuit and/or being shut off separately. The sudden buildup in pressure in the fuel to, for example, 60 bar causes the injection nozzle to open and fuel to be injected through the injection nozzle into a combustion space of an internal combustion engine during an extremely short time of, for example, one 1000th of a second.
These pump and nozzle systems, known from EP 0 629 265, comprise an electromagnetically driven reciprocating plunger pump 1 and the injection nozzle 2 (FIG. 1). These pump and nozzle systems have proven useful in particular in two stroke engines in which previously large amounts of pollutants were known to be given off as a result of scavenging losses and a high fuel consumption came about in that a high proportion of fuel was able to pass through the outlet conduit 3 in an unconsumed form, because in two stroke engines the overflow conduit and outlet conduit 3 are opened simultaneously. With the pump and nozzle systems described above, it was then possible to reduce the fuel consumption and the expulsion of pollutants drastically. In addition, the unquiet running of the engine, which was previously due to irregular ignition at low speeds, could now be virtually completely prevented. In this context, the fuel is injected directly into the combustion space 4 of a cylinder 5 for an extremely short time and, specifically, only when the outlet conduit 3 is largely closed. The control 6 for optimizing the pump and nozzle system is provided electronically via, for example, a microprocessor which controls the injection time and the quantity of fuel, the injection time for this being determined as a function of load, for example with a temperature sensor 7, a throttle valve potentiometer 8 and a crack angle sensor 9. The microprocessor expediently also controls the ignition system 10 of the plunger cylinder unit of the engine which is provided with fuel by the pump and nozzle system.
By virtue of these pump and nozzle systems, the hydrocarbon emission is drastically reduced in comparison with other two stroke engines, the running feature in particular at low rotational speeds, being at the same time significantly improved. Carbon monoxide and the oil fed for lubrication are also expelled in significantly smaller quantities so that a two stroke engine of this kind can be compared with a four stroke engine in terms of the emission values, but, nevertheless, has the high performance with low weight which is typical of a two stroke engine.
In the pump and nozzle systems described above, the fuel circuit space is formed by a pressure chamber and a delivery plunger or armature space, the pressure chamber being the partial space area separated off from the pressure space by a static pressure valve and in which the kinetic energy of the armature is transmitted to the fuel, the armature space being the partial space area in which the fuel which is displaced without resistance can flow during the accelerated partial stroke.
According to the known pump and nozzle systems, the armature space can be connected via a housing bore to a fuel flooding or scavenging device, so that fuel can be fed through this partial space area during the injection activity of the armature and/or during the starting phase of the pump and/or of the engine. This flooding or scavenging with, for example cool, bubble-free fuel causes fuel containing bubbles in the armature space to be removed, the armature space and its surroundings to be cooled and the formation of bubbles owing to the effect of heat and/or to cavitation to be largely suppressed.
Under particular conditions, in particular when the fuel is acted on by heat which can be produced in the pump and nozzle system during operation, for example as a result of electrical energy and/or armature friction or the like, bubbles may penetrate the pressure space. This can adversely affect the function of the pump and nozzle system, and in particular the injection process.
U.S. Pat. No. 5,351,893 discloses a generic type-forming fuel injection device which, with an electric linear motor, drives a pump plunger in a reciprocal to and fro movement. The plunger is a tubular element which is displaceably mounted in a pump chamber. At the, in the feed direction, front end of the pump plunger, a plug is provided against which the pump plunger strikes at the end of its delivery stroke, as a result of which a pump conduit which is arranged at the front, in the feed direction, of the plunger is shut off and the fuel located in it is acted on with a feed pressure. In this device, fresh fuel is fed through the tubular pump plunger to the pressure conduit, as a result of which the fuel feed path extends through the electromagnetic drive unit of the injection device.
DD-PS 213 472, in particular its FIG. 3, discloses a further fuel injection device which operates according to the energy storage principle and which has an electromagnetically actuated reciprocating plunger element which compresses a fuel located in a pressure conduit and sprays it out at an injection nozzle. The reciprocating plunger penetrates a low-pressure chamber which is connected to the pressure chamber by means of a small conduit, a non-return valve being arranged in the conduit. The low-pressure chamber is arranged adjacent to the drive unit of the injection device and has a diaphragm which is actuated by the reciprocating plunger element and which serves to feed fuel from the low-pressure chamber into the pressure conduit, the low-pressure chamber being fed fresh fuel directly. Since in each case only small partial quantities are transferred from the low-pressure chamber into the pressure conduit, the main quantity of the fuel located in the low-pressure chamber remains for a considerable dwell time in the low-pressure chamber in which the fuel is heated.
The object of the invention is largely to avoid the penetration of gas bubbles into the pressure space and in particular also the formation of gas bubbles in the pressure space of the pump and nozzle systems described at the beginning.
This object is achieved by means of the features of claim 1. Advantageous developments of the invention are distinguished in the subclaims.
The invention accordingly provides, in particular, a pressure chamber in which the energy stored in the armature and/or in the delivery plunger element is transmitted to the fuel, the pressure chamber being formed separately from the armature space or armature area by virtue of the fact that the valve which interrupts the displacement without resistance is arranged outside the armature space. As a result, the heat generated in the armature space is not transmitted directly to the pressure chamber, causing the heating of the fuel compressed during the injection process, and thus the risk of the formation of bubbles, to be significantly reduced. In addition, the pressure chamber is freely accessible and is directly provided with a fuel feed line, so that only xe2x80x9cfreshxe2x80x9d and thus cool fuel is located in the pressure chamber. For further cooling, the pressure chamber can be provided with cooling ribs, for example. In addition, the pressure chamber can have a small-volume design so that there is always only a small amount of fuel in the pressure chamber, and thus the risk of a high proportion of bubbles is already decreased.
In addition, owing to the small flood space with direct supply of fuel, it is also necessary to scavenge only small quantities of fuel.
The double or two-sided axial guidance of the armature leads to a reduction in friction brought about, for example, as a result of the tilting movement of the armature, which was previously possible, and thus to a reduction in the production of heat.
The functionally impairing effect of gas bubbles and/or the heating of the fuel are virtually excluded.
The double-sided axial guidance of the armature not only remedies the problems described above. In other known embodiments of the pump and nozzle systems it also leads to a simplification of the spatial shape, to the simplification and thus also homogenization of the physical shape and thus to the simplification of the assembly of the armature and/or of the pump, but also in particular also to the reduction of radial vibrations of the armature, said vibrations being possible in the known pump and nozzle systems owing to the merely one-sided axial guidance and to unavoidable and/or unnecessary play, which reduces the excessively high friction, between the armature outer surface and cylinder wall of the pump, and said vibrations adversely affecting the reproducibility of the injection processes.